Nope, I just have the first 4, buying them in order.
Hmmmm...Wkik'ing it, I see A Death in the Family is with Ace and Seven, where'd that come from? (Don't tell me if it's really spoilery)
Looks like I'll have to buy that one, and hang onto it until I get there
As Brendan said, listen to them in order. The timeline works something like this ...
Six with Peri ... Peri departs and marries the quiet guy
Six travels alone. And sometimes with Frobisher.
Six meets Evelyn, they travel together. She is keeping a massive secret from him, which you heard right at the beginning - she has a weak heart.
Older and wiser, Six changes his costume to an indigo version (the blue costume seen in "Real Time").
Evelyn decides to stop traveling. She makes the Doctor one of her famous chocolate cakes as a goodbye present. ("The Wrong Doctors")
Immediately after, Six decides it's time to pick up Mel and travel with her. He changes his costume back to the original technicolor dream version. ("The Wrong Doctors")
We've seen times before where the Doctor meets up with a past companion and has adventures (or just one) with them. Two Doctors, for example. City of Spires for another (Six with Jamie again). At some future point, Seven meets up with Evelyn and has another adventure with her. That's that story.
Other Six and Evelyn stories have come out since then ... just know that at some point in her future, she leaves. That happens with every companion sooner or later.